Already considered a major threat because it could wipe entire hard drives and render systems inoperable, KillDisk was recently observed adding encryption capabilities and behaving like ransomware. For that, the malware needed elevated privileges, registered itself as a service, and then killed various processes, although it avoided essential ones.

According to ESET, the malware is associated with a threat group dubbed TeleBots, which is believed to be an evolution of the Russia-linked BlackEnergy (Sandworm) group. The group supposedly targeted Ukraine’s financial sector with various tools, including a newer version of KillDisk set to become active after a specific period of time and to overwrite files that featured specific extensions.

The file-encrypting variant of KillDisk, which was detailed last month, was targeting Windows systems, encrypted files, and demanded $250,000 for the decryption key. ESET, however, stresses on the fact that, even if the victim pays the ransom, the chances that files would be recovered are very slim.

Moreover, the security researchers warn that KillDisk is now targeting Linux systems, including workstations and servers, which is likely to cause even more damage. The ransom message is similar with the one used in the Windows variant, and the same is true for the demanded ransom: it is still 222 Bitcoin, or around $250,000.

The Linux variant of the malware, researchers say, displays the ransom note in an unusual manner: within the GRUB bootloader. What this means is that the malware overwrites the bootloader entries to display the ransom text.

The main encryption routine recursively traverses multiple folders within the root directory up to 17 subdirectories in depth, while files in them are encrypted using Triple-DES applied to 4096-byte file blocks. The threat uses a different set of 64-bit encryption keys for each of the encrypted files, researchers say.

Just as before, however, KillDisk proves highly damaging, as the infected systems can no longer boot after reboot. What’s more, the encryption keys generated on the affected host aren’t sent to the command and control server and aren’t saved locally either, meaning that there’s no way of recovering the files. Thus, paying the ransom is a waste of both time and money, ESET warns.

“Let us emphasize that – the cyber criminals behind this KillDisk variant cannot supply their victims with the decryption keys to recover their files, despite those victims paying the extremely large sum demanded by this ransomware,” the security researchers note.

However, a weakness in the encryption employed in the Linux version of ransomware makes file recovery possible, albeit difficult, ESET says. However, the same does not apply to the Windows variant.

“The group (or groups) of attackers behind these operations has had an interest in various platforms – whether it was Windows PCs controlling SCADA/ICS systems, or workstations in a media agency. With this latest expansion, attackers can use KillDisk to destroy files on Linux systems. Nonetheless, any ties between orchestrators of these attacks remain unclear and purely circumstantial,” ESET says.

What’s unusual regarding KillDisk is the addition of ransomware-like capabilities, given that previous attacks were cyber-espionage and cyber-sabotage operations. In fact, the security researchers say that the added functionality was more likely meant to make the malware more destructive than to provide it with true ransomware features.

“Whatever the true explanation, our advice still holds – if you’ve become a victim of ransomware, don’t pay up, since there’s no guarantee of getting your data back. The only safe way of dealing with ransomware is prevention – education, keeping systems updated and fully patched, using a reputable security solution, keeping backups and testing the ability to restore,” ESET concludes.